1. Field of the Invention
The present invention relates generally to road working machines and, more particularly, but without limitation, to an apparatus for providing a portable elevation control reference.
2. Prior Art
When a road working machine having a work piece, such as a planing cutter, is utilized to perform a surfacing operation on a roadway, such as a surface planing operation, an elevation control reference may be disposed along the roadway and adjacent the work piece when the finished surface is to have a predetermined elevation. One of the more common means for establishing an elevation control reference is a conventional stringline setup positioned alongside the roadway. Sensing means on the road working machine continuously senses the predetermined elevation of the stringline above grade and provides an elevation control signal to elevation control means on the road working machine. In turn, the elevation control means maintains the work piece at a predetermined working elevation, or cutting depth, in relation to the grade of the roadway. Thus, a uniform finish grade can be formed which is substantially free of high or low spots. In some instances, an existing curb alongside the roadway may be utilized in place of a stringline as the elevation control reference, if the grade elevation of the curb is generally uniform in relation to that of the roadway.
Stringline setups have more than proved their worth when utilized as an elevation control reference for making an initial pass down the roadway. It is relatively simple to construct a stringline setup along the shoulder portion of a roadway in a manner convenient to the positioning of the sensing means. However, as subsequent passes are made along the roadway, it becomes highly impractical to utilize the initial stringline setup, and it is not economical to construct a separate stringline setup for each pass made by the road working apparatus. This is particularly true when the roadway is relatively wide.
There are also many occasions when a planing operation is required to remove a specified amount of material from an existing roadway. An example would be a planing specification that requires the removal of the surface to a specified depth measured in inches below the existing roadway surface.
Whether the roadway working machine is to remove surface material by referring to a stringline or to the existing surface itself, it would be desirable to have an elevation control reference which is portable, or which, more or less, is always positioned adjacent the sensing means. Although not meeting this description, walking beam assemblies connected alongside a road working machine have been utilized in some instances to provide an elevation control reference which is roughly indicative of the desired finish grade. A walking beam assembly of this latter type is disclosed in U.S. Pat. No. 3,414,327, issued to Austin.
In the Austin patent, a grade elevation sensor assembly is utilized for determining the average grade elevation of the roadway forward of the work piece. The grade elevation sensor assembly provides a control signal indicative of this average grade elevation, and the control signal does substantially maintain the work piece at a predetermined working depth. However, since the grade elevation sensor assembly senses the grade elevation of the roadway forwardly of the work piece, the work piece will generally be maintained at a working depth indicative of the average elevation forward of the work piece as opposed to the average grade elevation surrounding the work piece. Thus, the resulting finish grade formed via the work piece will not be as uniform as if the work piece had been positioned at a working depth indicative of the average grade elevation of the roadway near the work piece.
An additional problem with conventional portable walking beam assemblies is that they restrict the maneuverability of the road working machine. In the use of a road working machine employing a planing cutter, by way of example, the machine frequently must make multiple cutting passes along the roadway. At the end of each pass, the machine must be manuevered into position to make the next cutting pass. It is not unusual to find that the machine must be turned around and repositioned to travel in the opposite direction, and such turn around often must be performed in tight quarters having minimum clearance and maneuvering room. When a conventional portable control reference apparatus is attached to the road working machine, more turn around area must be provided in which the machine is repositioned, or the control reference apparatus must be removed before turning the machine, and remounted once the machine is repositioned.